DISCUSSION

Historically, two schools of thought existed regarding the systematic relationships and taxonomic status of the arvicoline forms closely related to, and currently nested within, Microtus. The first school was composed primarily of paleontologists (Van der Meulen, 1978; Repenning, 1983, 1992; Zakrzewski, 1985) who, based on the nature of the fossil record, relied on tooth morphology as their primary data source. The resultant paleontological perspective recognized two primary taxonomic groups. The first group, which was acknowledged explicitly as being paraphyletic with regards to the second, was characterized by three closed triangles on m1 and included the extant Pitymys (or Pedomys). The second group was recognized based on the presence of four or five closed triangles on m1 and was hypothesized to have evolved out of the three-triangle taxon. All forms in this second group were allocated to Microtus.

The second school, composed primarily of mammalogists (Hooper and Hart, 1962; Hooper and Musser, 1964; Anderson, 1985), mined characters from additional anatomical sources (such as the cranium and glans penis) not generally available to the paleontologist. The resultant neontological perspective, although not derived from our current conception of a phylogenetic topology, rejected the recognition of the three-triangle forms as a distinct taxonomic group and considered most, if not all, of these species to be Microtus. Some workers shifted between the two perspectives (Martin 1974, 1987).

Since the late 1980s a number of phylogenetic hypotheses based on molecular data have appeared for arvicolines that support the general conclusions of the second school (Chaline and Graf, 1988; Moore and Janeck, 1990; Conroy and Cook, 2000; Galewski et al., 2006). The topologies derived from these studies largely agree that the extant North American species of Microtus form a clade and that the extant North American three-triangled forms—M. pinetorum and M. ochrogaster (i.e., Pitymys, Pedomys)—evolved independently from taxa with m1s characterized by five closed triangles.

If the topologies of these recent phylogenetic studies continue to be supported and the general systematic community adopts a complex history of dental evolution for this group, what are implications for the ability of the fossil record, based on sites such as Fiene, to provide meaningful data regarding the evolutionary history of Microtus? This question is not trivial considering the historical importance of Microtus and other arvicolines to our understanding of Neogene biochronology and faunal dynamics. Only considering Fiene, a number of more specific questions are central to understanding the evolutionary history that produced the recovered fauna. Are the three-triangle M. llanensis and M. meadensis more closely related to five-triangled forms, as suggested for M. pinetorum and M. ochrogaster? Is Allophaiomys monophyletic and how are these forms related to Microtus? Is the m1 morphotype of Allophaiomys ancestral to the slightly more complex morphotype of M. llanensis or are we observing another form of variation as in Microtus pennsylvanics where the number of m1 triangles is correlated to the forms of available vegatation (Davis, 1987)?

All of these questions are dependent on the ability of paleontologists to place isolated teeth within a broad phylogenetic context—a context that includes molecular-based topologies. This ability may well be dependent on expanding our understanding, beginning with extant taxa, of new forms of dental data. These data likely will include the developmental dynamics and evolutionary histories of enamel microstructure, morphometric delineated shape change, and discrete character variability. The beauty of novel hypotheses is not the opportunity they provide to reject previous ideas and the data on which those ideas are based, but rather the stimulus they provide to ask new questions, seek out even more data, and to consider previous observations within a new context. Rather than depressing the importance of the fossil record, the emergence of new phylogenetic hypotheses, whether based on molecular or other forms of data, should serve as a catalyst for exciting new research opportunities in arvicoline paleontology.